1. Field of the Inventions
The present invention relates to an active noise control apparatus for a three-dimensional space and, more particularly, to an active noise control apparatus for effecting an active noise control apparatus at local areas in a three-dimensional area, such active noise control apparatus is suitable to suppress noises caused by periodic sound or random sound, such as electromagnetic noise, blade tone noise or the like of a fan disposed at an inlet port of an air conditioner in an active noise control manner in a three-dimensional space.
2. Description of the Related Art
Heretofore, in order to reduce noises of an air conditioner or the like, a passive noise control method has been adopted. The passive noise control method includes a noise source control by reducing a vibration which forms a noise source of an air conditioner, a noise absorption in which a noise produced is absorbed by a noise absorbing material, a noise shielding in which a noise produced is prevented from being transmitted to another area, or the like. Recently, an active noise control method has been developed. According to the active noise control method, a noise produced is controlled by another sound which has a same wavelength (same frequency) and same amplitude as those of the noise to be controlled. The active noise control method has come into practical use in the field where a noise is transmitted in a one dimensional space such as in a duct of an air conditioner.
The fundamental concept of the active noise control method itself has been known from about 1936. The active noise control method requires coincidental generation of a sound having a same wavelength (same frequency and same amplitude as those of a noise to be suppressed, which has not been realized until now.
Recently, a digital signal processing art has made a considerable development until it has become possible to generate a sound wave having a same wavelength (same frequency) and same amplitude as those of a noise to be suppressed and thus the active noise control method has become realized.
An example of a conventional active noise control method for a one-dimensional space is illustrated in FIG. 1. The active noise control system as shown in FIG. 1 includes a noise detecting means 1, a sound generating means 2, a noise control effect detecting means 3 and a controller 4. A noise is transmitted from an upstream side (left side in FIG. 1) through a duct (for example, an air conditioner duct) to a downstream side (right side in FIG. 1). The noise is detected by the noise detecting means 1 and is converted into an electric signal, which is fed to the controller 4. The controller 4 acts to analyze the electric signal and feed an output signal to the sound generating means, to effect the active noise control. The sound generating means radiates a sound having an opposite phase to that of the noise into the duct. The noise control effect detecting means 3 acts to detect a noise control effect produced by the sound generating means. The output signal fed to the sound generating means 2 is corrected by feeding back a signal from the noise control effect detecting means 3 to the controller 4, on the basis of the noise control effect, so that the noise is always suppressed at the position of the control effect detecting means 3. Thus, the noise is suppressed at the position of the noise control effect detecting means 3 and the downstream side thereof (the right side in FIG. 1). The controller 4 includes an adaptive filter 4A which can be varied according to an adaptive algorithm 4B by digital signal processing. The controller 4 further includes a fixed filter 5. In order to effect the digital signal processing, an actually measured value of a transfer function Ho, which is defined by a ratio of (a voltage V.sub.3 of an output signal from the noise control effect detecting means 3 to the sound generating means 2)/(a voltage V.sub.2 of an input voltage from the controller 4 to the sound generating means 2), as shown in FIG. 2, is fed to the fixed filter 5. In an actual measurement, sound signals of M-series are outputted as a white noise from the controller 4 and these sound signals are radiated from the sound generating means 2, while the sound signals are returned from the noise control effect detecting means 3 to the controller 4. The Ho is obtained from these sound signals. Thus, a distance between the sound generating means 2 and the noise control effect detecting means 3, reverberation characteristic in the duct and instrument characteristics of the sound generating means and the noise control effect detecting means are taken into consideration, and these factors can be corrected at the process of noise control.
An interference of sounds radiated into a three-dimensional field is different from that of sounds radiated into a one-dimensional field, in that the active noise control for the three-dimensional space is different from the active noise control for the one-dimensional space. Presuming an ideal state, if a sound generating means was located at the same position with that of a noise source and a sound having a same amplitude and same wavelength as those of the noise was radiated from the sound generating means, a destructing interference of sound waves would occur, so that a noise suppression in the three-dimensional space could be effected. However, in a real state, it is impossible to locate the sound generating means completely at the same position with that of the noise source and, therefore, in the active noise control for the three-dimensional space it is required to locate a plurality of sound generating means near the noise source (each at a distance at least shorter than 1/2 of a wavelength). In other words, the greater sound control effect can be obtained, as the wavelength of sound is longer (that is, the frequency is lower) and the sound source is smaller. This is because a sound coming from a noise source becomes similar to a non-directional sound coming from a spot source. Conversely, a smaller sound control effect can be obtained as the wavelength of sound is shorter (that is, the frequency is higher) or the sound source is larger, because the noise has directivity.
There are other problems in the active sound control for the three-dimensional space.
FIG. 3 illustrates an active sound control for a three-dimensional field into which sound waves having a same amplitude and same frequency but opposite phases to each other are radiated from two spot sound sources S1 and S2. The spot sound sources S1 and S2 produce sound wave W1 and W2. Solid lines and broken lines of the sound waves W1 and W2 indicate loops and nodes of the sound waves, respectively. In such state, owing to interference of the sound waves having a same amplitude, same frequency and opposite phases to each other, sound decreasing areas A where the two sound waves cancel out with each other and sound increasing area B where the two sound waves add to each other are formed. Accordingly, if one of the sound sources is a noise source and the other sound source is used as a sound generating means for the active noise control system, it is possible to effect the active noise control for the three-dimensional field. The size and the noise control effect of the local area where the active noise control can be effected are in proportion to the wavelength of the sound and, therefore, single or plural noise control effect detecting means, single or plural sound generating means and single or plural noise detecting means are provided, depending upon the position and size of the local area where the noise suppression or noise reduction is to be effected, the frequency of sound and the object of noise reduction, and the arrangement of such noise control effect detecting means, sound generating means and noise detecting means is determined according to the object. It is further required to pay attention to the space of the room, appearance as an interior, cost of each instrument and the like. It is an object of solving such various problems in the active noise control for the three-dimensional space.
Japanese Patent Application Laid-Open No. HEI-2-61450 discloses an air conditioner which includes a first sound receiving means, for receiving a noise produced from a fan or the like, a phase converting means for converting a phase of a waveform received by said first sound receiving means into opposite phase, a sound generating means for converting a waveform signal fed from said phase converting means into a sound, a second sound receiving means for detecting a composite sound in which the sound from said sound generating means is combined with the original noise and control means for discriminating levels of a sound received by said second sound receiving means and a sound received by said first sound receiving means, respectively, to effect level adjustment and phase adjustment, whereby said control means acts to effect noise control of said composite sound received by said second sound receiving means.
This noise control method is not technically unsatisfactory in order to effect the active noise control for the three-dimensional space.
The applicant has proposed an air conditioner which includes an active noise control apparatus for a three-dimensional space, as shown in FIGS. 4 and 5, in which a plurality of sound generating means are arranged near a noise source. FIG. 4 is a perspective view of the air conditioner which is of ceiling mounted type. In order to reduce a low frequency periodic noise, such as blade tone noise, electromagnetic noise on the like, which is radiated to a whole three-dimensional space, by an active noise control method, noise detecting means 1 (blade tone pickup, electromagnetic noise pickup) are disposed near a turbo-fan 6 which forms a noise source, and sound generating means 2a and 2b and noise control effect detecting means 3a and 3b are disposed around the turbo-fan 6. The apparatus further includes a controller 4, an air filter 7, a grill 8 and an electrical control box 9.
FIG. 5 illustrates a general control arrangement for effecting active noise control of the air conditioner as shown in FIG. 4.
According to the arrangement as shown in FIGS. 4 and 5, it is only possible to control a noise coming from a noise source which is of a non-directional property and it is difficult to control a noise having relatively high frequency in a three-dimensional space because it is required to control a noise having a directivity.
Furthermore, in the case of the air conditioner of an active noise control type, such problem that an air flow is impeded by sound generating means and other inconvenience tend to occur. It is further required to pay attention to lower the costs of the apparatus and to enable effective use of respective instruments for the active noise control.